1. Field of the Invention
The present invention relates to a wireless communication system, and more particularly to a method for performing handover between base stations in order maintain the communication link with a Mobile Station (MS).
2. Description of the Related Art
Next generation communication systems are being developed in order to provide various high-speed large-capacity services to MSs. A representative example of the next generation communication systems is the Mobile WiMAX (Worldwide Interoperability for Microwave Access) communication system, an example of which is a communication system based on the Institute of Electrical and Electronics Engineers (IEEE) 802.16 standards.
In general, the WiMAX mobile communication system is based on the IEEE 802.16e Wireless Metropolitan Area Network (WMAN) standard guaranteeing the mobility of an MS, and supports Broadband Wireless Access (BWA). Handover performed in the WiMAX mobile communication system includes soft handover, softer handover, hard handover, etc. As used herein, handover refers to a process that is necessary in accordance with movement of an MS to a new traffic channel when the MS moves from one BS to a new BS or to a new antenna coverage area within the area of one BS. During the handover, it is important that the transfer of the call be successful while preventing degradation of the communication quality. It can be said that the handover is a process of efficiently recognizing change in the communication environment and newly establishing a communication path to an optimum BS. That is, the handover corresponds to a process of establishing a communication path to a neighboring BS having a better communication environment than a serving BS when an MS is located at a cell boundary area.
The softer handover particularly refers to communication transfer from a BS currently in communication with a MS to a BS expected for the same communication with the MS without forced interruption of the existing communication channel. That is, the softer handover refers to a handover required when an MS currently receiving a service moves between sectors, which are divided from a cell to which a service is provided by a predetermined BS.
FIG. 1 illustrates a typical distributed antenna system that includes distributed antennas distributed around the same BS. In the distributed antenna system shown in FIG. 1, multiple distributed antennas 1, 2, 3, . . . , and N are distributed in a geometrically uniform arrangement around one BS 10. A MS (not shown) can communicate with the BS 10 through a distributed antenna corresponding to the area where the MS is located. Discrimination of the distributed antennas can be based on its own coverage area from a neighboring distributed antennas. The coverage areas of the distributed antennas altogether constitute an entire area, cell 20, controlled by a single BS.
FIG. 2 is a signal flow diagram of a handover process initiated by an MS in a typical WiMAX mobile communication system. Referring to FIG. 2, after initial network entry, a serving BS 202 transmits a Mobile Neighbor Base Station Advertisement (MOB_NBR_ADV) message to an MS 200 (step 210). The serving BS 202 transmits a DL_MAP/UL_MAP message to the corresponding MS 200 currently performing communication (step 212). While data is being exchanged, if the MS 200 determines to scan the neighbor BS 204, the MS 200 makes a request for a scanning interval by transmitting a scanning interval allocation request (MOB_SCAN-REQ) message to the BS (step 214). In response, the corresponding BS 202 transmits a scanning interval allocation response (MOB_SCAN-RESP) message including a scanning interval and scanning object information to the MS 200 (step 216). Then, the MS 200 scans the neighbor BS 204 within the scanning interval received from the serving BS 202 and measures signal intensities of the neighbor BS 204. When the signal intensity of the neighbor BS 204 is larger than that of the serving BS 202, the MS 200 reports this information by transmitting a mobile station handover request (MOB_MSHO-REQ) message to the serving BS 202 (step 218). The serving BS 202 accepts the handover determination (high speed BS switching/macro diversity handover), and requests resources by transmitting a handover notification (HO_NOTIFICATION) message to a target BS (step 220). In response, the target BS 202 transmits a handover notification response (HO_NOTIFICATION-RESP) message (step 222). Then, the serving BS 202 transmits a BS handover response (MOB_BSHO-RESP) message including information on the target BS 204 to the MS 200 (step 224), and the MS transmits a handover indication (MOB_HO-IND) message identifying completion of the handover to the serving BS 202 (step 226). The serving BS 202 transmits a handover notification confirmation (HO_NOTIFICATION-CONFIRM) message to the target BS 204 (step 228), and then transmits a DL_MAP/UL_MAP message to the MS 200 and the target BS 204 (steps 230 and 232). Thereafter, the MS 200 transmits an RNG_REQ message to the target BS 204 (step 234), exchanges an RNG_REQ/RESP message with the target BS 204, establishes a normal operation state, and then completes the handover process. Thereafter, the MS 200 prepares a process for the target BS 204. Specifically, the MS 200 exchanges data with the two BSs in the case of macro diversity handover.
FIG. 3 is a signal flow diagram of a handover process initiated by a BS in a typical WiMAX mobile communication system. Referring to FIG. 3, after initial network entry, a serving BS 302 transmits a Mobile Neighbor Base Station Advertisement (MOB_NBR_ADV) message to an MS 300 (step 310). The serving BS 302 transmits a DL_MAP/UL_MAP message to the corresponding MS 300 currently performing communication (step 312), exchanges data with the MS 300, and then determines to perform handover. The serving BS 302 transmits a handover notification (HO_NOTIFICATION) message to a target BS 304 (step 314). Further, the serving BS 302 transmits a DL_MAP/UL_MAP message to the MS 300 and the target BS 304 (steps 324 and 326). Thereafter, the MS 300 transmits an RNG_REQ message to the target BS 304 (step 328), exchanges an RNG_REQ/RESP message with the target BS 304, establishes a normal operation state, and then completes the handover process.
The conventional handover process as described above takes only the handover between BSs into consideration without reflecting handover in a distributed antenna system including a plurality of divided sectors within the same cell. In this regard, the BSs must synchronize with each other in relation to time and frame structure. However, such synchronization is a very difficult job, and a problem of overhead may occur while the BSs participating the handover share or transmit the MAC.